The invention relates to a fuel feed system for a spark-ignition internal combustion engine including a direct fuel injection valve mounted on a cylinder head for injecting fuel and to a method of operating an internal combustion engine with such a fuel feed system.
The prior art already includes a fuel feed system (EP 0 308 467B1) which has a common rail which is attached to a cylinder head and which carries both fuel and air under pressure to blow-in valves accommodated in the cylinder head. For this purpose, corresponding fuel lines and air lines to the blow-in valves are provided in the common rail. Each blow-in valve has an electromagnetically actuable fuel injection valve of conventional construction which can discharge fuel into a mixture formation chamber provided within the blow-in valve. The fuel in the mixture formation chamber is then prepared with compressed air and partially vaporized in the mixture formation chamber. The fuel prepared in this way passes on into a discharge zone of the blow-in valve which has an electromagnetically actuable valve. In an open position of a valve-closing body of the electromagnetically actuable valve of the discharge zone, the fuel/air mixture in the mixture formation chamber is then blown out directly into the combustion chamber from an outlet opening of the blow-in valve at a relatively low pressure of, for example, 7 to 8 bar. The common rail is provided for supplying the blow-in valves with both fuel and air.
However, if large injection quantities are demanded from the known fuel feed system or from the blow-in valve, as is the case, for example, with high engine loads, there may be problems with the quality of mixture preparation since there is only a certain maximum quantity of fuel that the blow-in valve can prepare well. The same problems are also encountered with a fuel feed system which has direct-injection valves for direct petrol injection. Direct-injection valves discharge the fuel directly at a relatively high pressure in the form of a cloud of fuel comprising extremely fine droplets into a combustion chamber of the internal combustion engine. If relatively large quantities of fuel are demanded, however, the fuel discharged by the direct-injection valve is not fully prepared and use of the air in the combustion chamber is not optimal. Complete preparation of the fuel/air mixture discharged by the blow-in valve cannot be achieved with blow-in valves either. All this means that wetting of the internal walls of the cylinder and of the piston surface with fuel can occur. However, this kind of thing leads to incomplete combustion of the fuel in the combustion chamber, leading to a rise in noxious components in the exhaust gas. Particularly affected by this are internal combustion engines which have pressure charging of the intake air, by means of an exhaust turbocharger for example, since, in relation to the cylinder volume, these internal combustion engines require relatively large quantities of injected fuel.
It is the object of the invention to provide a fuel feed system with which an optimum mixture preparation as regards consumption and exhaust emissions takes place in the internal combustion engine in all operating ranges.